while testing S4 AD DC I installed a wxp test box. It seems to join the domain ok (I get a welcome message) But after reboot I am told that the name (machine name?) already exist on the network and the domain oculd not be joined. Snooping the network I see nothing suspicious except for the last to packets:
192.168.2.33 -> 192.168.2.255 NBT NS Registration Request for BLA[0], Success
192.168.2.22 -> 192.168.2.33 NBT NS Registration Response for AKUMA[0], Active Error
192.168.2.22 being the S4 server, 192.168.2.33 being the client. Both client IP and hostname (bla) are uniq on the network. The S4 AD DC was freshly provisioned. Upon login I get a message that the domain was unreachable and could not be used. The debug output from the server shows incoming stuff from the client though - I even got a message that the machine account was authenticated. No funny stuff here as well.
The Windows7 client joins and works just fine with the same server (same settings, same database, different client ip/name)

(In reply to comment #1)
> We provide a very good support for Windows XP as a client. Might therefore the
> client itself be the problem? Do you have another Windows XP client ready for a
> join test?
the client was a fresh install of wxp pro sp2, no patches though (I just wanted to check if xp works). I'll try to find another xp box to test.